1. Field of the Invention
The subject invention relates to the processing of audio signals, and more particularly, to boosting the bas response of audio signals.
2. Description of the Related Art
It is common for audio systems to include controls for boosting and/or attenuating the bass response of the audio signal to suit the preferences of a user of the audio system. However, it has been found that while a particular adjustment may be satisfactory for low signal levels, as the signal level increases, the set amount of boost may cause overloading of the amplifier and/or damage to the loudspeakers and/or loudspeaker enclosures attached to the system.
European Patent Application EP 0 122 663 to Freadman discloses a method and system for improving speaker performance in which the magnitude level is detected for both low and high frequency portions of an input audio signal. Based on these detected magnitude levels, low and high frequency active voltage control shaper circuits generate control signals for a constant velocity equalizer to dynamically control the high and low frequency response such that at low input signal levels, the high and low frequency portions of the input signal are boosted, while at higher input signal levels, the high and low frequency portions are boosted to a lesser extent.
European Patent Application EP 0 554 962 to Laupman discloses tone control circuitry having a frequency characteristic dependent on the input signal level, in which the low frequency boost of the Freadman circuit is enhanced by coupling a fixed filter circuit to the Freadman circuit.
It is an object of the invention to provide a dynamic bass control circuit which is considerably simpler than the known systems.
This object is achieved in a circuit for providing a variable amount of bass control on an input signal dependent on a signal level of said input signal, said circuit comprising an input for receiving the input signal; high-pass filtering means coupled to said input, said high-pass filtering means having an output forming an output of said circuit; means for coupling the output of said high-pass filtering means to ground, said coupling means having a variable impedance in response to a control signal, wherein said coupling means varies a Q value of said high-pass filtering means; and means coupled to said input for detecting a signal level of said input signal, said detecting means generating said control signal for said coupling means in dependence on said detected signal level.
The fundamental operating principle of the subject invention is to vary the Q value of a second order high-pass filter so as to cause an increase in Q under low level input signal conditions, while causing a lowering of the Q of the filter with high level input signals. This is achieved by placing a control element in shunt across the output of the high-pass filter such that at low signal levels, the control element is open effectively removing the shunt allowing the filter to operate in its maximum Q condition. At increased signal levels, the control element is closed and the filter is loaded by the shunting resistance causing a lowering of the Q value and also increasing the cut-off frequency of the high-pass filter.